AC welding of SS316L

By Lawrence (*****)

Date 12-03-2011 14:58

Welcome to the forum Sargam.

AC does "not" have a good penetrating effect with the GTAW process when you compare it to DCEN

The Chromium oxide on 316 is not similar to aluminum oxide... AC GTAW uses the DCEP side of the AC cycle to promote cathodic bombardment, otherwise known as "cleaning action" This action blasts off aluminum oxide... ]

AC current also puts considerably more heat on the GTAW electrode than does DCEN... The AC weld pool is wider and shallower than DCEN, which has negative effects on most aspects of GTAW of stainless steels.

These are some of the reasons why... There are more.

By js55

Date 12-03-2011 15:41

Wow Lawrence. If you are ever to receive the credit you deserve for the knowledge you demonstrate you are going to have to wax pontificably more than taking 3 short statements to explain so clear and consicely that even I can understand it, as opposed to what I've read in technical articles taking 30 pages and being left even more confused than before. :grin:

By Lawrence (*****)

Date 12-03-2011 16:32

Some days you just don't have time to type out 30 pages..

By gndchuck (**)

Date 12-04-2011 00:41

Lawrence just talks so that the rest of us can understand, which happens to be very nice!

By Dualie

Date 12-07-2011 05:38

i wish Lawrence would have written the cliffs notes i used in college. IT would have been 1000X easier.

By 803056

Date 12-07-2011 17:22

I can't add to the accolades for Lawrence's writing. He's good!

However, I would like to add to his explanation if I can.

The type of current used influences the amount of heat that is available at the arc and influences the amount of penetration.

DC electrode negative puts more heat where it is needed, in the weld pool. The arc efficiency of DCEN is about 70%. The remaining 30% of the heat is wasted through the tungsten and the torch.
DC electrode positive has lower arc efficiency than DCEN. Only about 30% of the energy is used and 70% of the energy is wasted. The result is less penetration for the same energy used.
AC has an arc efficiency of about 50%, so half of the energy is wasted and half the energy goes into the weld pool. The penetration would be better than DCEP, but still not as effective as DCEN.

Best regards - Al

By MMyers

Date 12-07-2011 20:02

And to further add to what Lawrence and Al have already said,

Al's comments are absolutely true if you are using a machine with a 50% duty cycle AC wave without DC offset.

Screwing with the waveform (which you can do on alot of modern machines) doesn't change where the energy goes, so the EN and EP cycles still obey the rules Al outline in his post, but it does change the *total* amount of energy (heat) that goes into the work and into the tungsten/torch.

An AC wave that is 200 amps EN 90% of the time and 50 amps EP 10% of the time is a completely different animal with regard to heat and penetration than an AC wave that is 200 amps EN 50% of the time and and 200 amps EP 50% of the time.

By 357max

Date 12-14-2011 23:58

A good test is worth more than a thousand opinions.
If you have an inverter AC/DC - set the "Balance" or "Unbalance" the the AC (squarewave) sine wave so that 95 to 99% time is spent on electrode negative using the 5-1% at electrode positive. Make the weld on stainless steel or any other metal that would "normally/traditionally" be welded with DCEN - note the results. It is not illegal but maybe immoral to weld with an unbalanced AC on metals other than Al or Mg

By ozniek

Date 12-15-2011 13:39

OK 357max, you have got me really interested. What will you see if you did the test you described? (With the unballanced AC) Your use of the terms "not illegal" and "maybe immoral" must be the best sales pitches I have heard on the forum! I hope you are in sales, if not, you are missing your calling!

Regards
Niekie

By Lawrence (*****)

Date 12-15-2011 15:08 Edited 12-15-2011 15:18

Niekie

My "opinion" was baised on about a thousand tests :)... I just diddn't blather about it before because it wasn't relevant to the Original Posters question.. But now I'll say a word or two in order to avoid confusion and purchases of expensive equipment that will live their service lives running on settings that can be achieved by equipment 1/4 the cost.

I did this type of testing at United Airlines with scrap engine exhaust and hot section componants for GE CF6-6 and PW JT8D turban engines.

The thought was that sometimes when field repairs are made on exotics/superalloys like Inco 718, Hastelloy X, Waspalloy, and good ol 347 austinetic stainless steel, that from time to time it was simply impossible to remove the oxides from these hot section parts as well as it can be done at the depot level.

We had settings something like:

Inco 718 .060-.090 thickness

DCEP 200-250 amps
DCEN 60-90 amps
Balence 96-99% DCEN
AC Freq 200 Max.. No benefit above this.

A cathodic etch was clearly visible even on completely non-prepped surfaces. It did have a positive effect on oxide removal. Superior wetting at the toes compared to samples welded DCEN with no surface prep.

The catch is that those oxides, especially on the tail section of a jet, often have other hydrocarbons included from fuel, hydrolic fluid etc. These hydrocarbons will *NOT* be etched away.. They will be drawn into the soluable base metal in the form of Hydrogen, which is not generally a happy conclusion.

AC current (in any form) will do less cleaning than abrasives, chemical cleaning or a wire brush.. The reasons AC are used to break down aluminum oxide are not valid for stainless..

The "traditional" forms of surface prep and welding current selection for stainless is there for a reason... Some people also claim that high frequency pulsed DCEN current can "stirr" the weld puddle and produce improved results... My experience is that this will only benefit semi-automatic GTAW where travel speed is strictly governed and fitup and fixturing is more consistant.

If you have time to waste... You may enjoy the excercise as there is much to learn about harnessing the full capabilities of an Aysemmetric AC welding power supply.

By 357max

Date 12-15-2011 16:58

using 11 gage 304 with out any material prep, setting a Miller Dynasty at 50 amps, AC starting at 99% time at negative working to 95% negative, 120 hertz comparing DCEN at 50 amps. AC provided; a more base metal color match with less blues and darker straw color, a wider weld melt and quite good fusion, good wetting action without overheating, penetration was not attempted/measured with lap and tee joints. I'll try the square groove for penetration.
The new inverters with the 4 ac waveforms should be fun to experiment with also.